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Category: Michael Cassidy

In theory, thinning out traffic in carpool lanes should make traffic move faster. But kicking those drivers out of California carpool lanes had exactly the opposite effect, according to a new study by researchers at UC Berkeley’s Institute of Transportation Studies.

In July, 85,000 owners of hybrid vehicles in California lost the unique privilege of driving by themselves in carpool lanes. Federal law lets transportation agencies push those drivers out of carpool lanes if the average speed in those lanes dips below 45 mph during peak hours.

“Now we’ve got carpool lanes that are underused, and traffic is more congested,” said Michael Cassidy, professor of civil and environmental engineering, and co-author of the study. “This federal policy makes everything worse.”

Cassidy and doctoral student Kitae Jang analyzed six months of data collected from road sensors in the entire network of San Francisco Bay Area carpool lanes. The biggest surprise is that, even though only 1% of hybrid drivers were using the perk in the first place, taking away their ability to use carpool lanes increased traffic slowdowns overall by 10%.

“It surprised me, and I know how traffic works,” said Cassidy. “I thought it might be bad for regular lanes, but it turns out that it’s miserable for everybody.”

Metropolitan Transportation Commission (MTC) spokesman Randy Rentschler said that, although the study focused only on the Bay Area, he had no doubt that the data was correct.

“Traffic is a weird thing,” said Rentschler. “When the system is working at maximum capacity, very small changes can have very big effects.” He pointed to the use of metering lights to mitigate congestion as another example.

According to the study, the unexpected results were due to a concept called “dual influence.” Drivers in faster moving carpool lanes reduce their speeds to avoid collisions with frustrated slow lane drivers, who in turn cut in and out the carpool lane to try to get ahead.

The second factor slowing down traffic is fear.

“When the lane next to me is at 30,” said Cassidy. “I’m afraid to drive at 70.”

Rentschler said that the report’s assertion that letting more cars use the carpool lane improves performance in all lanes begs an important question: What’s the best way to get more vehicles into the carpool lanes?

“In this case it was the people who had the money to buy Priuses who were allowed into the lanes,” said Rentschler. “But what about people who can’t afford those cars?”

The report outlines several possible solutions, including a system in which all commuters take turns using the carpool lanes on different days, and the use of High Occupancy Toll lanes, where drivers pay a fee to use carpool lanes.